Tertiary Mammals and Continental Drift.A
Rejoinder to George G. Simpson.

by Alexander Du Toit (1944)

Editor Charles H. Smith's Note: Original pagination
indicated within double brackets. Notes are numbered sequentially and
grouped at the end, with the page(s) they originally appeared at the bottom
of given within double brackets. Reprinted by permission of the American
Journal of Science. Citation: American Journal of Science 242
(1944): 145-163.

[[p. 145]] ABSTRACT.
Doctor Simpson's criticisms are opportune, pertinent and helpful, although
partial, through being confined to the rôle of the mammals, while
ignoring the overshadowing geologic and tectonic evidence--Tertiary and
older. The Cretaceo-Tertiary Land-bridges of du Toit are the logical outcome
of the Earth's structural pattern whether Drift be admitted or not,
and must therefore be viewed as essentially orogenic in character and
generally linear, narrow, impersistent and imperfect, particularly during
their later history. Simpson has merely shown that such inter-continental
linkages could not have persisted as long, nor have permitted terrestrial
migration as freely, as believed, but he has by no means disproved the
probability of Continental Drift, which has been based on a wealth of
other evidence, much of it long antedating the evolution of the Mammals.

INTRODUCTION.

Critics of Continental
Drift are apt to forget that this conception--like the accepted science
of Geology--is still being shaped and must experience revisions and modifications
during its evolution, more especially in regard to inter-continental linkages
during the Tertiary. They are equally liable to ignore the fact that current
ideas on such linkages are often unhelpingly vague and too frequently
lack a strictly geologic basis.

When hypotheses
of Drift are being criticised, it is essential that the particular scheme
of former continental spacing and movement be specified, since there is
as yet no true consensus of opinion on the subject and several different
systems have thus far been advanced. The citations made from Wegener are
usually from the English translation of the third German edition of 1922,
regardless of the fact that several revisions have appeared thereafter,
the latest being the French version, dated 1937, of the fifth and last
German edition. (9) In this latter the chapter on Paleontologic and Biologic
Arguments embodies much new matter as well as extensive quotations from
recent literature, all of which the reader is earnestly invited to study.
For that reason this particular aspect will only be dealt with briefly.

[[p. 146]] Realising
the value to the problem of specialised knowledge, the writer issued in
1937 an invitation to authorities to coöperate, a challenge which
has been accepted by Dr. G. G. Simpson, speaking on behalf of Tertiary
Vertebrate Paleontology. In a trenchant article in The American Journal
of Science entitled "Mammals and the Nature of Continents," (4) he
has set out to prove that the evidence of mammalian distribution, while
limited in scope and time, is adverse to hypotheses of transoceanic or
drifting masses, but favorable to current views of stable continents.
In contravention it may be questioned whether the mammalia have not created
almost more biogeographic difficulties than they have solved. Even Matthew
(3; p. 299) is forced to admit a discordance between the vertebrate and
invertebrate evidence. Regrettably they have been robbed of much of their
value through their late appearance on the scene, well after continental
rupturing is deduced to have occurred, yet while such supposed movements
were in full swing.

Since Simpson
refers at length to the writer's particular views, but has at times failed
to present them correctly, or has omitted their other vital aspects, it
becomes necessary to go into greater detail than would otherwise have
been necessary, so as to clear up important differences of statement or
interpretation.

It is true that
paleontologists have thus far not shown any leanings towards the new ideas
and seemingly remain "orthodox" in outlook, but such is understandable
for, as the writer remarked in 1927 (6; p. 118), "geological evidence
almost entirely must decide the probability of this hypothesis for those
arguments based upon zoö-distribution are incompetent to do so."
After studying Simpson's paper and consulting those authorities cited
therein that are available in South Africa, the author sees no reason
for modifying that apparently sweeping pronouncement. For argument, the
finding of identical mammalian species in, say, Madagascar and Rhodesia
would merely indicate a former land-connection, but in the absence of
other information would not prove whether Madagascar had at the time lain
10 or 1000 miles distant from Africa or had been attached thereto via
Natal, Mozambique, East Africa or Somaliland. Those common fossils would
only say "yes" and "when," but not "where," or not with the precision
desired. Furthermore, as contrasted with other methods, paleontologic
comparison suffers from [[p. 147]] certain
disabilities, as set forth below. Within those limitations excellent use
can be made of this valuable arm, and in the case of the mammalia, from
the Eocene onwards. Perhaps the above may help to explain the remarkable
paradox that, while paleontologists as a body are antagonistic towards
Drift, the protagonists can claim that their views are actually supported
by paleontologic data. For that matter Simpson has done signal service
by pointing out serious errors not only in initial identifications or
comparisons, but in the perpetuation of those mistakes through lack of
critical study of the original sources, in which the writer must incidentally
admit to having shared.

Study of the many,
varied and conflicting opinions expressed by more than one generation
of biogeographers reveals most strikingly the serious difficulties, surprises
and dilemmas encountered by them, which collectively leave a strong impression
that some elusive, though vital, principle must have been missing from
their particular ideas of past continental linking. It is more than significant
that, of those welcoming the hypothesis of Drift, so goodly a number have
been biologists. One of the exceptions is Matthew (3; pp. 202-9), who
practically eliminates land-bridges and is seemingly content to explain
faunal relations on the Noah's ark principle, though even so he runs into
difficulties. The notion of random, and sometimes two-way, "rafting" across
the wide oceans, which he favors, evinces, however, a weakening of the
scientific outlook, if not a confession of doubt from the viewpoint of
organic evolution.

PALEONTOLOGIC HANDICAPS.

a) Paleontologic
Record. The "imperfection" of the record is universally deplored.
Everywhere, though more particularly in the southern lands, paleontologic
knowledge has lagged, and must continue to lag, far behind geologic. Not
only are the terrestrial less known than the marine Tertiaries, but they
have been, as Matthew points out (3; p. 178), eroded from over wide areas,
and the gaps in our knowledge concerning their life must be considerable,
whereas the corresponding marine faunas are relatively well known. Conclusions
from the meager finds recorded must largely be tentative. Furthermore,
unlike present-day life, the scanty vertebrate fossils have in the main
been collected from few and widely separated deposits in which the recorded
proportions of the determinable genera and species could be very different
from their actual frequency in [[p. 148]]
nature. Such assumes importance when faunal resemblances are tabulated
on a percentage basis, as done by Simpson (4; p. 20). The scantiness of
such material cannot be overlooked nor the likelihood of unexpected discoveries
that would appreciably modify current ideas. Negative evidence must be
quite unreliable therefore.

b) Instability
of Determinations. Although writing without "special competence"
in this field, the author has been struck by the fragmentary nature of
so much of the material labeled generically or specifically, the periodic
changes in nomenclature brought about by subsequent revision, the departures
in opinion regarding affinities expressed by men of eminence, etc., so
well exemplified by Simpson's account of the history of Neotragoceros
(4; pp. 24-5). The observer is led to speculate inhowfar the genera and
species listed to-day, and of course their affinities, would remain appreciably
unaffected by future work and better material. All this is apart from
that fundamental question, which cannot be discussed, "What actually constitutes
a Species?"

c) Wrongful
ascription. To the examples quoted by Simpson can be added the incorrect
identification of forms because of their association with admittedly related
fossils. One example of outstanding importance will be cited, namely the
recording by Amalitzky in 1899 of typical members of the Glossopteris
Flora together with "Karroo Reptiles" in the Permian of Russia, an association
that has never been questioned, at least in print. While the Karroo affinities
of the Russian vertebrates are beyond doubt, the writer's examination
in 1937 of several of the types identified as Gangamopteris and
Glossopteris by Amalitzky, or of duplicates so labeled by him,
showed that those particular specimens, while outwardly very similar,
yet failed to agree in their venation with those type genera of the Glossopteris
Flora. That Amalitzky should have been so misled is under the circumstances
quite excusable, but this instance gives rise to speculation whether other
analogous cases of paleontologic inexactitude might not exist unsuspected.

d) Specific
Determination. In the early days of research specific determinations
in far-off lands were, and understandably, made with the well known forms
of Europe. With progress and the realisation of faunal provinces, the
institution of local species became inevitable, a policy which may perhaps
have been overdone. A disturbing psychologic factor, which [[p.
149]] is quite distinct from the "personal equation" must here
be pointed out: namely the natural tendency to compare new finds with
material described from corresponding formations nearest to hand. It is
probably difficult to keep this "distance factor" under subjection, since
any tendency to make specific comparison with rather similar forms from
half way round the globe could so readily be regarded as incautious, with
the contingency that the closer relationships of far-separated faunas
might for long escape notice. In not a few cases again the more distant
faunas, particularly if the relevant literature be in some foreign language,
are liable to be overlooked, with the same ultimate result.

With the continents
viewed as no longer fixed, this aspect becomes one of actual concern,
and indeed, if the feasibility of drift be conceded, reconsideration
of the long-distance correlation of faunas generally would appear
to be inevitable. Until a considerable amount has been done in that direction
the existing paleontologic evidence can be accepted only with reserve
by those favoring drift.

FAUNAL RELATIONSHIPS.

The degree of
similarity between any two faunas unquestionably presents great difficulties.
Simpson has exposed several weaknesses in the writer's statements and
arguments on this subject, some of which are frankly admitted, due primarily
to the inherent tendency to stress the points of similarity for the very
reason that the areas concerned happen to be so far apart to-day. Such
relative accentuation finds a parallel when a paleontologist speaks of
"highly fossiliferous Cambrian" when he means "highly fossiliferous for
the Cambrian." Simpson criticises the writer's statement "2) faunal differences
at the ends of a link, as von Ubisch (quoted by Wegener) has pointed out,
ought to be appreciable owing to the differentiation that must have occurred
along the bridge itself, whereas a close specific relationship or even
identity of forms may indeed characterise the life of the now-opposed
lands," in which certain, and not all, forms were so obviously indicated.
He then says "If, as the logical alternative, he means to say that identical
faunas have been found on separate continents, then he is even farther
from the truth," thereby enlarging the idea of certain closely specific
or identical forms (not faunas) into one of identical faunal
assemblages.

[[p. 150]] With
no desire to detract from the true value of paleontologic comparisons,
but the weaknesses exposed in the preceding section in mind, a protagonist
of drift may be pardoned for feeling some reasonable doubt whether the
mathematical comparisons of similar or related species of distantly-spaced
faunas, as compiled from current literature by Simpson (4; p. 20), Arldt
or others, are as real as he is asked to believe. The instance cited of
the apparent lack of identical species among the Triassic reptiles of
Brazil and South Africa should not strictly be pressed in view of the
limited amount of collecting, the high ratio of new genera and species
so far found, and conceivably the "distance factor." Even under the closest
fitting of the continents permissible, the distance between the two areas
could not have been less than 1300 miles.

Accepting, however,
as a basis of discussion Simpson's percentages (4; p. 20) as applied to
faunal relationships, they but serve to emphasise the rapid falling
off of specific similarities with distance, even within the confines
of a single landmass, a peculiarity which is quite well marked among the
living faunas. With a long and narrow linkage the "con-specific ratio"
falls to a surprisingly low figure to become negligible or zero in extreme
cases, even when a past union of the particular lands happens to be indicated
by other credible evidence. Striking is the lack of common species from
among the closely allied Permian reptiles of Russia and South Africa,
areas 6500 miles apart.

Now the Cretaceo-Tertiary
land-bridges envisaged under Drift (see next section) are essentially
linear or arcuate, narrow and unstable, ranging in character, it is thought,
from continuous corridors to isthmian links in the sense of Bailey Willis.
In not a few instances they would have paralleled Simpson's Case D with
a community of families of 67 per cent, of genera 24 per cent and of species
5 per cent, while the respective percentages might well have been lower.
In certain instances such bridges would have run with the Earth's climatic
zones (Behring Strait; Central Atlantic); in others across them (Malaya-Australia-New
Zealand-Antarctica). For post-Eocene times they could in general have
corresponded closely with the "sweepstake routes" so aptly defined by
Simpson. Even in the case of the Afro-Eurasiatic connections (Spain-Morocco;
Egypt-Syria) the limited stratigraphic data are not necessarily in favor
of wide bridges across the Mediterranean "Tethys."

[[p. 151]] Under
those circumstances his contention, that a far higher proportion of common
species should be detectable under Drift than is brought out by the recorded
fossil mammalia, is greatly weakened. It may be questioned too whether
the mammals, by their very mobility and peculiar organisation, could be
expected to have migrated with the same presumed uniformity as the ancestors
of the living lower orders--snakes, scorpions, earth-worms, land and fresh-water
mollusks, plants, etc.--which, because of their number and variety are
surely of not less importance in this problem, especially as their evidence
proves to be not always in accord with that of the vertebrates (3; p.
299). In his latest edition (9; Chap. VI) Wegener effectively quotes authorities
such as Michaelsen, von Ubisch, Okland, Huus, Jaschnov, Meyrick, Irmscher,
etc., in those other fields.

Despite criticism,
the statement "Migration along a link need not be equally effective in
both directions" (7; p. 294) is reaffirmed, a cogent illustration being
that of Russia and Southern Africa with similar reptiles, amphibians and
fresh water mollusks, but different plants during the Permian. Simpson
admits (4; p. 17) possible selection during intermigration between continents
now separated, though only along limited corridors. Conceivably, with
only a very brief spell of linking, something rather like "one-way traffic"
could have operated; of such Madagascar would have formed an example.
The factors controlling migration must be extremely complex, and, in the
case of crustal wrinkles thrust up from the ocean-bed or lavas and scoriae
heaped up to above sea-level the laws of biologic spreading within continental
areas might not hold good. With fixed continents round-about routes have
often had to be postulated by biogeographers--to well within the polar
circles sometimes; these, if biologically possible, must have tended towards
the elimination of the less adaptable forms, and thus towards selection.

Simpson has had
to pass over the abundant non-mammalian life and thus ignore so much of
the very evidence that has compelled biogeographers to formulate their
individual schemes of past land-connections. Of such living forms admittedly
many, probably most of them, have as yet no well known fossil representatives,
although of high significance none the less. To argue that such southern
disjunctive distribution is due to colonisation from the north through
forms not yet discovered in the Holarctic region, is neither scientific
nor fair. Attention [[p. 152]] can merely
be drawn to the remarkable distribution of certain organisms, as done
by Wegener and others, not forgetting their parasites (7; p. 294). The
plants too have received insufficient attention, seeing that their thermal
sensitivity makes them useful through indicating, for example, the difficulties
inherent in the assumption that the cold temperate floras of the southern
continents, which show unexpected resemblances, must necessarily have
been derived from the Holarctic vegetation that flourished north of the
climatic barrier set by the wide Tropical Zone. The same argument applies
to the fresh-water decapods.

LAND-BRIDGES.

The way of the
bridge-builder is strewn with pitfalls. Simpson deplores that, opponent
as the writer is of the numerous land-connections of current conception,
he (du Toit) has nevertheless accepted several bridges differing from
the latter, so it is stated, in no respects, and faulty too in that they
could only be broken once, whereas paleontologic evidence would suggest
a repeated "make and break."

That is very far
from being the case, since under the hypothesis proposed by the writer
such connections--save possibly in the extreme northern Atlantic--are
viewed as having been essentially of an orogenic kind, developed
within restricted zones of lateral compression, produced by one crustal
block impinging on a second, or else thrown up from an epicontinental
sea or even from an ocean by marginal or advance folding in front of a
drifting block or blocks. Longitudinal tension and/or erosion, or sinking
in the post-orogenic period, would have served to reduce and even to sever
such connections. The geologic record is punctuated by such far-extended
squeezings (7; p. 48), the Tertiary being in fact marked out by at least
three world-wide compressive phases and interludes.

This "segmental
oscillation," which Simpson cannot accept, is written large, though in
other characters, in the Tertiary fold-girdles, which should dispose of
his complaint that under Drift the "make and break," so rightly stressed
by him, could not have been cyclic. Is such not perhaps Joleaud's idea
of the "accordion movement" under a different guise?

More important
still is the fact that such "fold-" or "drift-linkages" have not been
flung out at haphazard, but at spots and times closely fixed by the visible
and dated crumplings of the crust, and have not been conjured up, as Simpson
would [[p. 153]] hint, to explain actual
or supposed resemblances of the terrestrial life of the continents in
question. Indeed the situations of such "bridge-heads" are regarded as
pre-determined, their prolongations across the lands being in
most cases actual major fold-ranges. Even under current views such fold-zones
are regarded by a large body of geologists as persisting beneath the present
oceans and as having at times in the past projected as island-chains along
certain stretches thereof. Accepting Drift, they are merely shorter to
start with. It can be asked, and rightly, why, if such be the case, there
should be any recourse to Drift. The bitter fact is that current Geology,
sublimely unconscious of its impotence, is wholly unable with the continents
remaining fixed to account for the tectonics of those fold-zones in a
physical, logical and convincing fashion! For an adequate explanation
some drift would have to be invoked.

Of no less importance
through ruling out any bridge-head is the presence along a coast-line
of fringes of purely marine strata--or even their probable former presence,
if adequately supported by other lines of evidence. As an example, the
little-disturbed Lower Cretaceous to late Tertiary marines bordering the
Southern Atlantic exclude from consideration lengthy stretches of African
and American shores. On the contrary widespread unconformities or thick
non-marine intercalations point to uplift and to possible temporary connections.
Paleogeographers have all too frequently overlooked such basic restrictions
when locating their bridges.

By the writer
the Hypothesis of Drift is regarded as essentially established by the
Paleozoic and early Mesozoic evidence; the rest is largely a logical corollary,
which in the main is consistent in outline, but has not yet been worked
out in detail, for that would be a mighty task. The continents are pictured
as having proceeded to "part" during the later Mesozoic as shallow gulfs
were extended into and between them under the initial crustal stretching,
but actual rupture of the sial shell did not generally take place until
the close of the Cretaceous, to pass through a series of climaxes during
the Tertiary. Such should dispose of the erroneous idea that at the beginning
of the Tertiary the continents were still usually in contact along their
edges--making allowance for the widths of the continental shelves--with
opportunity for free biological migration.

The true rôle
of the mammalia will be the fixing of land- [[p.
154]] bridges not in the horizontal, but in the vertical plane,
through revealing the uprisings and downwarpings of the crests of such
linkages, themselves positioned by other, and mainly older, criteria.

THE NORTHERN CONTINENTS.

That those lands,
including Africa and India, had some inter-migration during the Tertiary
is proved by the mammalian evidence marshalled by Simpson and needs no
stressing. His arguments (4; pp. 8-10) in favor of a connection across
Behring Strait are indisputable, while his recognition of its apparently
cyclic nature is in agreement with the known triple folding that crosses
from Siberia to Alaska, and is not in conflict therefore with the scheme
of linking advocated by the writer (7; pp. 186, 298).

On the most vital
part of the problem, namely the connection between North America and Europe,
Simpson firmly opposes all bridges across the Central Atlantic but concedes
some linking farther north, possibly of late date (4; p. 6). Now the bridging
of this ocean, with a span of between 900 and 1500 miles across its narrower
part, much of it over 1000 fathoms deep, while perhaps a simple matter
for the biologist, is a far tougher proposition for the geophysicist,
who cannot so readily admit the sinking of non-tectonic crustal blocks
of that width in defiance of isostatic principles. Only a narrow linkage
of either tectonic or volcanic origin can accordingly be visualised. Outside
the presumably local wrinklings of southern England and western Spitzbergen
no relics of Tertiary fold-ranges are represented along the North Atlantic,
while the Tertiary faulting, e.g. Greenland and the British Isles, although
common, is nowhere to our knowledge of truly large vertical displacement.
Under current views the only assumption remaining would seem to be a volcanic
chain extending say from the British Isles via the Faroes and
Iceland to Greenland and Canada.

Under Drift the
North Atlantic is regarded as having been widened by intermittent tension,
a movement least and slowest in the north, with magma erupting from fissures
produced in the stretching ocean floor, and building up piles of volcanic
material that were in turn ruptured towards the close of drifting, a process
that may still be in progress. The problem is admittedly more or less
equally explicable under theories of stable or moving continents, but
the latter has the merit of forming the logical consequence of a particular
scheme of [[p. 155]] Earth structure which
seems to hold good for the pre-Tertiary eras. The practical outcome, however,
is that, when current views are applied to the Northern Continents, the
assumptions required are not any simpler, or more probable from the geophysical
standpoint.

AMERICAN-EURAFRICAN LINKAGES.

Simpson's exposure
of the flimsy nature of Joleaud's paleontologic evidence for Tertiary
Central Atlantic connections is welcome, and the writer candidly admits
his lapse through citing Gregory in this matter. All the same, the feasibility
of double linkages, of an orogenic type, between Central America, Cuba
and Spain and Venezuela and Morocco respectively during the Cretaceo-Tertiary,
happens to be a consequence of the scheme of drift proposed and is furthermore
independent of any paleontologic similarities between those four opposed
continents.

Those bridges
are regarded as having been pressed up under the rhythmic approach of
the North American-Eurasiatic and the South American-African masses, but
lowered by lengthwise tension under the westerly drift of the Americas
as the meridional "rift" of the Atlantic widened. Their persistence above
sea-level must have depended essentially on the relative intensities both
in space and time of the periodic but contrasted north-south compressions
and east-west tensions. The shallow-water marine faunas of those latitudes
show that for the earlier Tertiary the state amounting to a line of submarine
ridges was undoubtedly attained, while certain resemblances in the terrestrial
life, referred to below, suggest that the state of island-chains, or even
of isthmuses, was reached. The feasibility of effective transoceanic connections
after the Miocene was, however, doubted by the author (7; p. 204) and
he should therefore have ruled out as a possible migrant so much younger
a genus as Hipparion. South Africa has nevertheless yielded Hipparion-like
teeth, though their specific characters would appear not incompatible
with a northern derivation. While the affinities of the mid-Tertiary fauna
of South-west Africa are according to Stromer with North and Central Africa,
the specialized hyracoid Protypotheroides is allied to the Miocene
Protypotherium of Patagonia.

Conceding these
two structural "fronts"--Central America-Spain and Venezuela-Morocco--approaching
each other while being stretched unequally at their western ends, accompanied
[[p. 156]] by their attendant advance and
diagonal folds, the migrational paradoxes of the Central Atlantic that
have been forced upon biologists, and that Simpson has found (4; p. 25)
so hard to credit, become easier to understand. Indeed, with such long,
sinuous, warping and disrupting chains quite unexpected interchanges of
terrestrial life could hardly have failed to take place, as remarked by
H. B. Baker. As illustration, ignoring Drift, taking a chart of the Caribbean
region and delineating simple rises or falls of sea-level, peculiar sunderings
or linkings of the various islands would follow, to become more surprising
still were even a little crustal warping to be introduced into the process.
One should not overlook too the tremendous part played by volcanicity
in the building up of the Tertiary ranges, so finely displayed for example
in the island-chains of the West and East Indies. Cross-connections or
short-circuits might thus be established, as in the case of the Lesser
Antilles.

Even with the
continents fixed geologic evidence is against any direct connection between
North and South America from the Mesozoic down to the late Miocene. How
then were the marsupials of South America derived from a pre-Tertiary
stock in Europe? The South American monkeys too in their brain development
are surprisingly similar to those of the Old World. Each truly demands
an early Tertiary trans-Atlantic bridge! While Simpson has been so far
successful in disproving any free intermigration of the mammalia across
the mid-Atlantic during the latter half of the Tertiary, there remains
in favor of an earlier bridge or bridges, as well claimed by numerous
biogeographers, quite a number of lower animals and plants, the distribution
of which forms a stubborn problem under views of fixed continents.

AUSTRALIA-ANTARCTICA-SOUTH AMERICA.

While Simpson
severely questions the mechanics advocated by the writer in regard to
these continents, a study of du Toit's restoration (7; Fig. 7) and of
an artificial globe will show that the maximum displacement of Australia
would not have been much more than 2000 mules. Even assuming his extreme
proposition, that all such movement had been concentrated into the post-Oligocene
period, Australia in order to reach its present position would only have
had to "speed" at under six inches per annum. There is also the alternative
that, although the parent mass had been broken into its three parts at
an earlier [[p. 157]] date, intermittent
contacts could still have been maintained between those portions by the
circum-Pacific Tertiary fold-zone running through Patagonia, Grahamland,
West Antarctica, New Zealand, New Guinea and the East Indies, which last
is a global fact and quite independent of any special theory of Earth
structure. The existence of such a lengthy ridge finds support from the
great resemblances of the Miocene marine littoral faunas of Patagonia,
Australia and New Zealand, as stressed by van Ihering, Ortmann and Hedley.

The curving of
that fold-zone around Australia shows how that block, twisting anti-clockwise,
attempted to by-pass Indo-Malaya, thereby permitting the Asiatic wave-front
to press out into the Indian Ocean with the development of crustal swirls,
while the Australian wave-front escaped into the Pacific. Conditions tending
towards the production and destruction of linkages between Asia and Australia
were accordingly to hand. Umbgrove (8; p. 34) has recorded the surprising
shallowness of the seas over the vast East Indies region throughout the
Tertiary--in strong contrast to the Mesozoic--with intense folding during
the mid-Miocene, large parts having certainly been land in the Eocene
and Pliocene. The intensity and rapidity of the Pleistocene movements,
that led to the upheaval of coral reef-terraces for many hundreds of feet,
to the tearing out of astonishingly deep oceanic basins, etc., have been
fully set forth by the Dutch geologists, although little known generally.
Under the universal lowering of sea-level in the Pleistocene New Guinea,
Australia and much of the East Indies --"Sundaland"--must for a time have
become joined by low-lying ground.

As against all
this evidence paleontologists and biologists alike have tended to stress
the biologic isolation of Australia from the early until the very late
Tertiary, a relationship which must surely have been less likely with
the continents fixed than with Australia far away and only drifting towards
Asia. In regard to the marsupials--and in such the writer stands corrected--Simpson
denies any special resemblance between those of South America and Australia,
though he does not appear to be so happy about the family Dasyuridae or
Thylacinidae of the order Polyprotodontia and the one advanced order Diprotodontia1,
possessed by both continents. [[p. 158]]
Suggestively their respective parasites are identical according to Breslau
(9; p. 105). He reluctantly admits that, giving that evidence its maximum
weight, any southern connection could only be "something of the order
of discontinuous evanescent island chains." One is tempted to speculate
whether any part is played by the "distance factor."

The contrasted,
yet homologous relations of South America and North America on the one
hand and of Australia and Asia on the other can advantageously be considered
together. Marsupials appeared in the Mesozoic in North America and Europe,
but neither they nor their Tertiary descendants seem as yet to be known
from Asia or Africa despite the fact that all those lands were interconnected
during the early Tertiary. Now according to Simpson they could not have
reached South America or Australia from the north, since those continents
were under his, as well as under current, views then unconnected with
the northern lands. Had such been the case those southern continents should
logically have received other northern vertebrates. Strikingly, early
Tertiary marsupials have not yet been recorded from Asia or Australia
though their future discovery there is not denied. For the present, until
the paleontologist can explain away such paradoxes, he can scarcely maintain
that the balance of existing knowledge negatives any Tertiary connection
between South America and Australia--from both of which Southern Africa
had already become detached.

Admittedly in
those two continents the resemblance among the marsupials only extends
to families and those lands are not therefore con-specific, while the
monotremes are absent from South America and the edentates from Australia.
Accepting Drift, however, Australia could never have been closer to South
America than 3000 miles--as measured across Antarctica--from either side
of which central mass those continents ultimately broke away. Any very
close faunal resemblances between those far-sundered and isolated lands
could well be lacking, an explanation anticipated by Wegener (9; p. 106).

The significance
of other terrestrial life should on the other hand not be overlooked,
such as the presence of giant birds, fossil or living in the "Gondwanaland"
region--in Patagonia, Seymour Island, Antarctica, South Africa, Madagascar,
Australia and New Zealand--while the distribution of the fresh-water fishes
is highly suggestive (Eigenmann). Striking are [[p.
159]] the fresh-water crayfishes of South America, Australia, New
Zealand and Madagascar--but not found in Africa--separated as they are
by the equatorial zone from those of the northern hemisphere (Ortmann).
Matthew's explanations are far from satisfying. The scorpion family Bothriuridae
is restricted to South America and Australia (Hewitt). Those curious worms,
the Phreodrilidae, are confined to the colder parts of the South Temperate
Zone, including Kerguelen and the Falkland Islands. The botanical affinities
are much more marked between South America and Australia than between
either of them and Africa (Hooker, Hemsley, Bentham and Andrews).

In the face of
the above faunal relationships, which could be greatly extended, there
would seem to be a prima facie case for a former linking or linkings
between South America and Australia via Antarctica during the
Cretaceo-Tertiary, an opinion stoutly maintained by not a few eminent
biologists and geologists.

MADAGASCAR.

In regard to the
separation of this island from Africa the mechanics are not so puzzling
as Simpson would suggest (4; pp. 27-9). Whether Drift be accepted or not,
the controlling factor was clearly the lengthy geosyncline, developed
in the Paleozoic, that ran down the eastern margin of Africa and covered
the western half of Madagascar. Various geologists have contributed towards
the elucidation of its history. The formations along both sides of the
Mozambique Channel dip towards the latter and indicate through their dominantly
marine and occasionally terrestrial phases the deepenings and shallowings
of that trough from the Lower Permian onwards, with intermittent but progressive
lengthening, until in the late Cretaceous it penetrated to the South Atlantic
(see 7; Figs. 11 & 12).

The affinities
of the later Cretaceous marine faunas in this region show that, while
this trough was in existence, peninsular India must have been prolonged
southwards to Madagascar to form a joint barrier such that mollusks from
the Mediterranean and North-west India were able to enter western Madagascar
only in limited numbers but failed to reach round to the Madras area and
beyond, where a different fauna held sway. Marine older Tertiaries fringe
western Madagascar dipping towards the Channel, and the island must then
have [[p. 160]] been cut off from Africa,
but stratigraphic gaps at the Oligocene and about the Miocene show that,
through slight shallowing of the geosyncline at those times, Madagascar
could again have become linked to the mainland. Its geologic history during
the Pliocene is unknown, though during the Pleistocene the sea-level fluctuated
more than once. Simpson's statement (4; p. 27) that "it contains early
Tertiary groups without their later Tertiary relatives abundant on adjacent
continents, mid-Tertiary groups with neither their early nor their late
Tertiary allies, and late Tertiary to Recent groups such as are always,
elsewhere, accompanied by members of other groups [are] here quite lacking,"
is quite consistent with the stratigraphic evidence.

From the data
available the writer has deduced the intermittent stretching of the crust
during the Jurassic and Cretaceous with slow widening of the geosyncline
(7; Fig. 12) and the outpouring of lavas, its actual rupture in the Eocene,
the gradual drifting of the Madagascar block to the south--in the wake
of Antarctica--with tilting to the west-north-west along with vast extrusions
of lavas in East and Northeast Africa and the Deccan as Australia and
India drew away from Africa. In the case of Pemba Island its separation
from the mainland is set by Stockley (5; p. 239) as late Miocene or early
Pliocene. Any closer dating of those separations would have to be furnished
by the terrestrial fossils, and among them the mammalia, though the evidence
provided is not only limited but confusing. From its home opposite Kenya
and Tanganyika--duly fixed by abundant and consistent data--Madagascar
would have had to travel about 600 miles, though in a direction largely
parallel to the mainland, a point of high importance when possible re-connections
with the latter are in question.

An Upper Cretaceous
connection between South America, Madagascar and India is suggested by
the dinosauria (von Huene; Matley), though Simpson does not explain how
that evidence can have been misinterpreted. Even allowing for isolation
and lack of knowledge concerning its fossil representatives, the living
Malagasy fauna remains as much a puzzle as ever. Matthew (3; p. 203) views
the insectivore Centetidae as the earliest colonisation, perhaps pre-Tertiary,
and the lemurs, rodents and viverrines as about mid-Tertiary. On the basis
of the more primitive types of lemurs Gregory (1; p. 372) [[p.
161]] regarded Madagascar as isolated from India by the Oligocene,
before India had been occupied by the more advanced lemurs. After the
beginning of the Tertiary the contacts with Africa were manifestly slight
and any linking must have been, as suggested earlier, brief.2

H. F. Standing
has shown that among the lemurs of Madagascar there are closer analogies
with the monkeys of South America than with those of the Old World, which
brings out the fact that there are positive resemblances with South America,
as long pointed out by Wallace, Lydekker, Gregory and many others, which
are explicable under the particular scheme of linking discussed earlier.
To mention only a few cases there are the boas, iguanas and certain of
the earth-worms (Acanthodrilidae) which suggestively are unknown north
of the equator.

Matthew (3; pp.
203, 206-7), whom Simpson follows with approval (4; pp. 28-9), explains
such peculiar relationships as due to occasional introductions from Africa
by "rafting," an hypothesis which in view of the limited distances involved--400
miles across the narrowest part of the present channel--is quite conceivable.
If true, there was apparently no return rafting, while the African monkeys
obviously failed to make the journey. The fact, however, that analysis
of the Malagasy fauna shows it to embrace diverse elements, which, as
Simpson himself points out, would seem to mark out distinct phases of
the Tertiary, while those in turn can be correlated with deduced crustal
movements in that region, all forces one to associate such elements with
terrestrial linking rather than chance rafting. One can even doubt the
alleged natatory abilities of the Malagasy hippopotami (Lemoine) or the
dinosaurs (Matthew). The flora of Madagascar does not provide much help,
being strictly endemic and much diversified, but demands a closer study
from botanists.

To sum up, the
faunal relationships between Madagascar and Africa prove not less puzzling
with fixed than with moving [[p. 162]] continents,
though explicable by the latter hypothesis under the reasonable assumption
of very brief post-Cretaceous connections. Those with South America and
Australia are difficult to interpret with stable continents since in several
critical cases either the ocean or the equatorial zone would have prevented
migration from the Holarctic region. On the contrary the relationships
would follow logically from Drift.

CONCLUSIONS.

Doctor Simpson's
positive assertion, that "the known past and present distribution of land
mammals cannot be explained by the hypothesis of drifting continents,"
can be met with a not less emphatic contradiction.

Anyone who sponsors
a new hypothesis must unhesitatingly investigate every conflict with facts
and modify his views accordingly, and Simpson's contribution--long overdue--is
therefore valuable through clearing away sundry errors and misapprehensions,
reviewing faunal relations from a new angle and throwing fresh light on
a complex and fascinating subject. At the same time he does not seem to
have appreciated the multitudinous and weighty geologic evidence upon
which had been based the writer's synthesis and in which several of the
inherent difficulties had been anticipated. Throughout his paper too he
has entirely ignored the vital stratigraphic and tectonic aspects.

What he has really
succeeded in proving is that du Toit's Tertiary Orogenic Linkages could
not have persisted above the level of the widening oceans down to so late
a period as assumed, or that they could not have been continuous enough
to permit migration with the freedom hitherto supposed. This consequence,
which is fully accepted, clarifies the position and marks a real advance
in Tertiary paleogeography.

On the contrary
he has not thereby disproved the Hypothesis of Drift, which is regarded
as resting upon a far wider basis--one rooted in Paleozoic times at least--while,
paradoxically, most of his statements of facts prove, when analysed, far
from incompatible with the idea of moving continents, and in particular
with the writer's scheme, modified slightly to meet such objections. It
is re-iterated that, only under Drift can unstable, though ordered, land-bridges
be made available at about the particular places and times required for
the inter-migration of terrestrial life--linkages which in addition give
[[p. 163]] the minimum number of possible
routes and which automatically exclude from consideration a host
of other connections that have been postulated, largely on paleontologic
or biologic grounds, though without full recognition of possible conflicting
geologic or other evidence.

The acid test
of that Hypothesis will, it is felt, depend among other things on its
ability simply and logically to account for the harvest of fossil forms
yet to be unearthed. In the meantime fresh evidence of every kind and
new ideas will be as welcome as ever in our attempt to establish on a
firmer basis this great revolutionary and far-reaching theory of Earth
Evolution, which we owe to the genius of Taylor and Wegener.

1. The sole South American genus Caenolestes
could admittedly be an aberrant polyprotodont, as believed by Broom and
Diderer. [[on p. 157]]2. The writer's reference to Gregory
queried by Simpson (4; p. 28 footnote) comes from (1; pp. 312 and 372)
and applies to the Afro-Malagasy mass; that ascribed to Lydekker (2; p.
1009) arose from his cryptic remark that the Malagasy fauna "is much more
distinct from the Ethiopian fauna than is the latter from the fauna of
either the Oriental or the Holarctic region." [[on
p. 161]]